GRAIN-ORIENTED ELECTRICAL STEEL SHEET

Abstract

Provided is a grain-oriented electrical steel sheet that achieves both low iron loss and low magnetostriction and has excellent transformer properties. The grain-oriented electrical steel sheet has a thermal strain-imparted region extending linearly in a direction crossing the rolling direction, and in the strain distribution in the rolling direction of the thermal strain-imparted region, the strain at both ends of the thermal strain-imparted region is tensile strain larger than the strain at the center of the thermal strain-imparted region.

Claims

1. A grain-oriented electrical steel sheet having a thermal strain-imparted region extending linearly in a direction crossing a rolling direction, wherein in a strain distribution in a rolling direction of the thermal strain-imparted region, strain at both ends of the thermal strain-imparted region is tensile strain larger than strain at a center of the thermal strain-imparted region.

2. The grain-oriented electrical steel sheet according to claim 1, wherein in the strain distribution in a rolling direction of the thermal strain-imparted region, a difference between an average of strain amounts at both ends of the thermal strain-imparted region, which is indicated as A, and a strain amount at a center of the thermal strain-imparted region, which is indicated as B, is 0.040% or more and 0.200% or less, where the difference is indicated as ?AB and is obtained by ?AB =A?B.

3. The grain-oriented electrical steel sheet according to claim 2, wherein the ?AB is 0.050% or more and 0.150% or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In the accompanying drawings:

[0057] FIG. 1 schematically illustrates candidate locations for forming a magnetic domain with a magnetization component in a different direction from the rolling direction in a steel sheet material that has been subjected to non-heat-resistant magnetic domain refining, which are used in the studies leading to the present disclosure;

[0058] FIG. 2 schematically illustrates an example of a ring-shaped beam profile;

[0059] FIG. 3 schematically illustrates an example of a strain distribution in a thermal strain-imparted region of the grain-oriented electrical steel sheet of the present disclosure;

[0060] FIG. 4 illustrates the relationship between the difference in strain amount ?AB (=A?B) and the material iron loss W.sub.17/50;

[0061] FIG. 5 illustrates the relationship between the difference in strain amount ?AB (=A?B) and the transformer noise level; and

[0062] FIG. 6 illustrates the relationship between the difference in strain amount ?AB (=A?B) and the transformer building factor.

DETAILED DESCRIPTION

Grain-Oriented Electrical Steel Sheet

[0063] The following describes suitable embodiments of the present disclosure in detail.

<Chemical Composition of Grain-Oriented Electrical Steel Sheet>

[0064] The chemical composition of the grain-oriented electrical steel sheet of the present disclosure or a slab used as the material thereof is a chemical composition capable of secondary recrystallization. In the case of using an inhibitor, for example, Al and N are added in appropriate amounts when using an AlN-based inhibitor, and Mn and Se and/or S are added in appropriate amounts when using a MnS/MnSe-based inhibitor. Of course, both an AlN-based inhibitor and a MnS/MnSe-based inhibitor may be used together.

[0065] In the case of using an inhibitor, preferable contents of Al, N, S and Se in the grain-oriented electrical steel sheet or a slab used as the material thereof are as follows, respectively. [0066] Al: 0.010 mass % to 0.065 mass %, [0067] N: 0.0050 mass % to 0.0120 mass %, [0068] S: 0.005 mass % to 0.030 mass %, and [0069] Se: 0.005 mass % to 0.030 mass %.

[0070] An inhibitor-less grain-oriented electrical steel sheet in which the contents of Al, N, S, and Se are limited may be used in the present disclosure. In this case, the contents of Al, N, S and Se in the grain-oriented electrical steel sheet or a slab used as the material thereof are preferably suppressed as follows, respectively. [0071] Al: less than 0.010 mass %, [0072] N: less than 0.0050 mass %, [0073] S: less than 0.0050 mass %, and [0074] Se: less than 0.0050 mass %.

[0075] The following describes the basic components and optionally added components of the grain-oriented electrical steel sheet of the present disclosure or a slab used as the material thereof in detail.

C: 0.08 Mass % or Less

[0076] C is a basic component and is added to improve the microstructure of a hot-rolled sheet. When the C content exceeds 0.08 mass %, it is difficult to reduce the C content during the manufacturing processes to 50 mass ppm or less where magnetic aging does not occur. Therefore, the C content is preferably 0.08 mass % or less. Because secondary recrystallization occurs even in a steel material containing no C, there is no need to set a lower limit for the C content. Therefore, the C content may be 0 mass %.

Si: 2.0 Mass % to 8.0 Mass %

[0077] Si is a basic component and is an element effective in increasing the electric resistance of steel and improving the iron loss properties. Therefore, the Si content is preferably 2.0 mass % or more. On the other hand, when the Si content exceeds 8.0 mass %, the workability and the sheet passing properties may deteriorate, and the magnetic flux density may also decrease. Therefore, the Si content is desirably 8.0 mass % or less. The Si content is more preferably 2.5 mass % or more. The Si content is more preferably 7.0 mass % or less.

Mn: 0.005 Mass % to 1.0 Mass %

[0078] Mn is a basic component and is an element necessary for improving the hot workability. Therefore, the Mn content is preferably 0.005 mass % or more. On the other hand, when the Mn content exceeds 1.0 mass %, the magnetic flux density may deteriorate. Therefore, the Mn content is preferably 1.0 mass % or less. The Mn content is more preferably 0.01 mass % or more. The Mn content is more preferably 0.9 mass % or less.

[0079] In addition to the basic components listed above, Ni, Sn, Sb, Cu, P, Mo, and Cr may be used as appropriate in the present disclosure as optionally added components, which are known to be effective in improving the magnetic properties.

[0080] That is, the grain-oriented electrical steel sheet or a slab used as the material thereof may suitably contain at least one selected from the group consisting of [0081] Ni: 0.03 mass % to 1.50 mass %, [0082] Sn: 0.01 mass % to 1.50 mass %, [0083] Sb: 0.005 mass % to 1.50 mass %, [0084] Cu: 0.03 mass % to 3.0 mass %, [0085] P: 0.03 mass % to 0.50 mass %, [0086] Mo: 0.005 mass % to 0.10 mass %, and [0087] Cr: 0.03 mass % to 1.50 mass %.

[0088] Among the above optionally added components, Ni is useful for improving the microstructure of a hot-rolled sheet and improving the magnetic properties. When the Ni content is less than 0.03 mass %, the contribution to magnetic properties is small. On the other hand, when the Ni content exceeds 1.50 mass %, secondary recrystallization becomes unstable, and the magnetic properties may deteriorate. Therefore, the Ni content is desirably in a range of 0.03 mass % to 1.50 mass %.

[0089] Among the above optionally added components, Sn, Sb, Cu, P, Mo and Cr are also elements that improve the magnetic properties like Ni. In any case, when the content is less than the lower limit, the effect is insufficient, and when the content exceeds the upper limit, the growth of secondary recrystallized grains is suppressed, resulting in deterioration of magnetic properties. Therefore, the content of each of Sn, Sb, Cu, P, Mo and Cr is preferably in the range described above.

[0090] The balance other than the above components is Fe and inevitable impurities.

[0091] Among the above components, C is decarburized during primary recrystallization annealing, and Al, N, S, and Se are purified during secondary recrystallization annealing. Therefore, the contents of these components can be reduced to the level of inevitable impurities in a steel sheet after secondary recrystallization annealing (a grain-oriented electrical steel sheet after final annealing).

<Manufacture of Grain-Oriented Electrical Steel Sheet (Before Forming Thermal Strain-Imparted Region)>

[0092] The grain-oriented electrical steel sheet of the present disclosure can be manufactured with the following procedure before the formation of a thermal strain-imparted region.

[0093] A steel material (slab) of a grain-oriented electrical steel sheet with the chemical system described above is subjected to hot rolling and then subjected to hot-rolled sheet annealing as required. Next, cold rolling is performed once or twice or more with intermediate annealing performed therebetween to obtain a steel strip with a final sheet thickness. The steel strip is then subjected to decarburization annealing, applied with an annealing separator mainly composed of MgO, then rolled into a coil, and subjected to final annealing for the purpose of secondary recrystallization and formation of forsterite film. If necessary, the steel strip after final annealing is subjected to flattening annealing, and then an insulating coating (such as a magnesium phosphate-based tension coating) is formed. In this way, a grain-oriented electrical steel sheet before the formation of a thermal strain-imparted region can be obtained.

<Formation of Thermal Strain-Imparted Region>

[0094] Next, a thermal strain-imparted region is formed in the grain-oriented electrical steel sheet. A thermal strain-imparted region can be formed by non-heat-resistant magnetic domain refining, which is one type of magnetization refining. In the non-heat-resistant magnetic domain refining, for example, an energy beam is applied to the surface of the steel sheet after final annealing or after the formation of an insulating coating to locally introduce thermal strain (to form a thermal strain-imparted region).

Method of Applying Energy Beam

[0095] During the formation of a thermal strain-imparted region, the strain distribution of the present disclosure can be formed more effectively by using an energy beam having a circular (ring-shaped) intensity distribution as seen in a ring-mode laser system.

[0096] The beam source of the energy beam may be a laser, an electron beam, or the like, any of which may be used to obtain the desired strain distribution. In the case of using a laser, a ring-mode laser system may be employed. In the case of using an electron beam, a circular (ring-shaped) convex portion may be formed on the cathode surface. In this way, the strain distribution of the present disclosure can be formed.

Direction of Applying Energy Beam

[0097] During the manufacture of the grain-oriented electrical steel sheet of the present disclosure, a thermal strain-imparted region can be linearly formed in the steel sheet by applying the above-described energy beam such as an electron beam.

[0098] Specifically, one or more electron guns are used to introduce linear thermal strain (form a thermal strain-imparted region) while applying the beam so as to cross the rolling direction. The scanning direction of the beam is preferably in a range of 60? to 120? with respect to the rolling direction, and in this range, it is more preferable to make the direction 90? with respect to the rolling direction, that is, to scan along the sheet transverse direction. This is because when the deviation of the scanning direction from the sheet transverse direction increases, the amount of strain introduced into the steel sheet increases, resulting in deterioration of magnetostriction properties.

[0099] The energy beam may be applied continuously along the scanning direction (continuous linear irradiation) or may be applied by a repetition of stopping and moving (dot irradiation), as long as the other requirements of the present disclosure are satisfied. Both irradiation forms can provide the effects of improving the building factor and the magnetostriction properties of the present disclosure.

[0100] Note that both the continuous linear and the dot described above are forms of linear.

[0101] The following is a more detailed description of suitable conditions for applying an electron beam during the manufacture of the grain-oriented electrical steel sheet of the present disclosure.

Accelerating Voltage: 60 kV or More and 300 kV or Less

[0102] As the accelerating voltage increases, the electrons move more and more straightly, and the thermal effect on an area where the electron beam is not applied decreases. Therefore, the accelerating voltage is preferably high. For this reason, the accelerating voltage is preferably 60 kV or more. The accelerating voltage is more preferably 90 kV or more, and still more preferably 120 kV or more

[0103] On the other hand, a too high accelerating voltage renders it difficult to shield X-rays formed by the application of the electron beam. Therefore, the accelerating voltage is preferably 300 kV or less from the viewpoint of practice. The accelerating voltage is more preferably 200 kV or less.

Spot Diameter (Beam Diameter): 300 ?m or Less

[0104] As the spot diameter decreases, it is easier to locally introduce strain. Therefore, the spot diameter is preferably small. The spot diameter (beam diameter) of the electron beam is preferably 300 ?m or less. The spot diameter (beam diameter) of the electron beam is more preferably 280 ?m or less and still more preferably 260 ?m or less. Note that the spot diameter refers to the full width at half maximum of a beam profile obtained with a slit method using a slit with a width of 30 ?m.

Beam Current: 0.5 mA or More and 40 mA or Less

[0105] The beam current is preferably small from the viewpoint of beam diameter. This is because, as the current increases, the beam diameter tends to increase due to Coulomb repulsion. Therefore, the beam current is preferably 40 mA or less. On the other hand, a too small beam current cannot provide sufficient energy to form strain. Therefore, the beam current is preferably 0.5 mA or more.

Electron Beam Power: 300 W or More and 4000 W or Less

[0106] The electron beam power is calculated as the product of the accelerating voltage and the beam current. Considering the amount of strain introduced, the electron beam power is preferably small. This is because increasing the electron beam power leads to excessive strain introduction, which deteriorates the hysteresis loss properties more than it improves the eddy current loss properties, and also deteriorates the noise properties. Therefore, under conditions where the accelerating voltage and the beam current satisfy the above suitable ranges, the electron beam power is preferably 4000 W or less. On the other hand, a too small electron beam power cannot provide sufficient energy to form strain. Therefore, the electron beam power is preferably 300 W or more.

Degree of Vacuum in Environment of Applying Beam

[0107] An electron beam is scattered by gas molecules, causing, for example an increase in beam diameter and halo diameter and a decrease in energy. Therefore, the degree of vacuum in an environment where the beam is applied is preferably high, and the pressure is desirably 3 Pa or less. The lower limit is not particularly limited. However, a too low degree of vacuum increases the cost of a vacuum system such as a vacuum pump. Therefore, the degree of vacuum in an environment where the beam is applied is desirably 10.sup.?5 Pa or more in practice.

[0108] The following is a more detailed description of conditions for applying a laser during the manufacture of the grain-oriented electrical steel sheet of the present disclosure.

Laser Power: 20 W or More and 500 W or Less

[0109] Considering the amount of strain introduced, the laser power is preferably small. This is because increasing the laser power leads to excessive strain introduction, which deteriorates the hysteresis loss properties more than it improves the eddy current loss properties, and also deteriorates the noise properties. Therefore, the laser power is preferably 500 W or less. On the other hand, a too small laser power cannot provide sufficient energy to form strain. Therefore, the laser power is preferably 20 W or more.

<Strain Property in Grain-Oriented Electrical Steel Sheet>

Strain Distribution

[0110] A strain distribution in the rolling direction of the thermal strain-imparted region on the surface of the steel sheet may be measured with the EBSD-Wilkinson method. In the EBSD-Wilkinson method, for example, an electron beam is applied on the surface of the steel sheet, Kikuchi pattern is obtained at each measurement point, and the strain amount is calculated based on the deformation amount of the Kikuchi pattern at each point using analysis software such as CrossCourt with a strain-free point as a reference point.

[0111] The thermal strain-imparted region in the present disclosure refers to the same region as a linear closure domain region formed by the energy beam linearly applied on the steel sheet. The length in the rolling direction of the closure domain formed on the surface of the steel sheet (the same as the length of the thermal strain-imparted region) can be measured by obtaining a magnetic domain pattern on the surface of the steel sheet using a commercially available domain viewer.

Average Strain Amount A and Strain Amount B

[0112] The strain distribution in the rolling direction of the thermal strain-imparted region on the surface of the steel sheet is measured with the above measurement method, and the average of the strain amounts at both ends in the rolling direction of the thermal strain-imparted region is indicated as A, and the strain amount at the center of the rolling direction of the thermal strain-imparted region is indicated as B. The strain amounts at both ends in the rolling direction may be the same or different.

[0113] When the difference between the A and the B, which is ?AB (A?B), is positive (exceeding 0.000%), the effect of the present disclosure can be obtained. When the difference is 0.040% or more and 0.200% or less, a grain-oriented electrical steel sheet with better properties can be obtained. The ?AB is more preferably 0.050% or more. The ?AB is more preferably 0.160% or less.

EXAMPLES

[0114] The following describes the present disclosure based on examples. The following examples merely represent preferred examples, and the present disclosure is not limited to these examples. It is possible to carry out the present disclosure by making modifications without departing from the scope and sprit of the present disclosure, and such embodiments are also encompassed by the technical scope of the present disclosure.

[0115] In this example, a slab having a chemical composition containing the components listed in Table 1 with the balance being Fe and inevitable impurities was used as a material of a grain-oriented electrical steel sheet. The slab was subjected to hot rolling, hot-rolled sheet annealing, cold rolling once, decarburization annealing, annealing separator application, and final annealing in the stated order and under predetermined conditions, respectively, to obtain a steel strip of a grain-oriented electrical steel sheet with a thickness of 0.23 mm.

[0116] [Table 1]

TABLE-US-00001 TABLE 1 Content (mass %) C Si Mn Ni Al N Se S O 0.08 3.4 0.1 0.01 0.026 0.007 0.011 0.003 0.0025

[0117] The steel strip of the grain-oriented electrical steel sheet was used as a sample, and the sample was irradiated with an energy beam. Either a laser or an electron beam was used as the beam source of the energy beam (as listed in Table 2), and the irradiation was either continuous linear irradiation or dot irradiation (as listed in Table 2). In this way, a thermal strain-imparted region was formed on the surface of the steel strip of the grain-oriented electrical steel sheet (magnetic domain refining treatment). The dot irradiation refers to a form of irradiation in which the energy beam is applied by a repetition of stopping and moving in the scanning direction.

[0118] The conditions of applying the energy beam, for both laser and electron beam, were as follows: direction of applying the energy beam: approximately 90? with respect to the rolling direction, and beam power: 0.6 kW to 6 kW (accelerating voltage: 60 kW to 150 kV, and beam current: 1 mA to 40 mA). In the case of electron beam, the degree of vacuum in an environment where the beam was applied was 0.3 Pa. The beam to be applied in both cases had a ring-shaped profile, and a beam with a beam diameter of 200 ?m was used. To change the values of the average strain amount A, the strain amount B, and the ?AB, the beam was applied by adjusting conditions such as the beam power, the energy difference between the energy local maximum value in the ring-shaped profile and the energy local minimum value at the center of the profile, and the distance between the energy local maximum values.

[0119] A sample was cut out from the steel strip of the grain-oriented electrical steel sheet in which a thermal strain-imparted region had been formed, and the magnetic flux density (B.sub.8) and the iron loss (material iron loss: W.sub.17/50) were measured as magnetic properties with the single sheet magnetic measurement method described in JIS C2556. In addition, a 3-phase stacked transformer (iron core mass 500 kg) was prepared with the steel strip, and the iron loss (transformer core loss: W.sub.17/50 (WM)) was measured at a frequency of 50 Hz when the magnetic flux density in the iron core leg portion was 1.7 T. The transformer core loss W.sub.17/50 (WM) at 1.7 T and 50 Hz was taken as a no-load loss measured using a wattmeter. With the value of the W.sub.17/50 (WM) and the value of the W.sub.17/50 measured with the single sheet magnetic measurement method, the building factor (BF) was calculated using the following formula (1). The results are listed in Table 2.

Building factor=W.sub.17/50(WM)/W.sub.17/50(1)

[0120] Further, a 3-phase transformer model for transformer was prepared using the grain-oriented electrical steel sheet that had been subjected to the magnetic domain refining treatment as described above. The transformer model was excited in a soundproof room under the conditions of a maximum magnetic flux density Bm of 1.7 T and a frequency of 50 Hz, and the noise level (dBA) was measured using a sound level meter. The results are listed in Table 2.

[0121] In the same manner as described above, a sample was cut out from the steel strip, and the strain distribution in the rolling direction around the thermal strain-imparted region was measured with the EBSD-Wilkinson method. Further, the length in the rolling direction of the closure domain formed on the surface of the steel sheet (the same as the length of the thermal strain -imparted region) was measured using a commercially available domain viewer (MV-95 manufactured by Sigma Hi-Chemical, Inc.). The average of the strain amounts at both ends of the thermal strain-imparted region (average strain amount) was indicated as A, and the strain amount at the center of the thermal strain-imparted region was indicated as B. The difference between the strain amounts ?AB (=A?B) was calculated. Note that tensile strain was positive, and compressive strain was negative. These values are listed in Table 2.

TABLE-US-00002 TABLE 2 Noise Beam Irradiation A B ?AB W.sub.17/50 level BF No. source form [%] [%] [%] [W/kg] [dBA] [] Remarks 1 Laser Continuous linear 0.050 0.050 0.000 0.700 38.0 1.42 Comparative Example 2 Laser Continuous linear 0.050 0.030 0.020 0.700 38.0 1.30 Example 3 Laser Continuous linear 0.050 0.020 0.030 0.700 38.0 1.30 Example 4 Laser Continuous linear 0.050 0.010 0.040 0.700 35.0 1.25 Example 5 Laser Continuous linear 0.050 ?0.025 0.075 0.695 34.0 1.25 Example 6 Laser Continuous linear 0.100 0.000 0.100 0.695 34.0 1.25 Example 7 Laser Continuous linear 0.100 ?0.050 0.150 0.695 34.0 1.25 Example 8 Laser Continuous linear 0.130 ?0.070 0.200 0.700 35.0 1.25 Example 9 Laser Continuous linear 0.160 ?0.090 0.250 0.700 38.0 1.30 Example 10 Laser Dot 0.055 0.055 0.000 0.695 35.0 1.40 Comparative Example 11 Laser Dot 0.055 0.035 0.020 0.695 35.0 1.25 Example 12 Laser Dot 0.055 0.025 0.030 0.695 35.0 1.25 Example 13 Laser Dot 0.055 0.015 0.040 0.695 32.0 1.23 Example 14 Laser Dot 0.055 ?0.020 0.075 0.690 31.0 1.23 Example 15 Laser Dot 0.110 0.010 0.100 0.690 31.0 1.23 Example 16 Laser Dot 0.110 ?0.090 0.200 0.695 35.0 1.25 Example 17 Laser Dot 0.140 ?0.110 0.250 0.695 35.0 1.25 Example 18 Laser Dot 0.170 ?0.130 0.300 0.695 35.0 1.25 Example 19 Electron beam Continuous linear 0.060 0.060 0.000 0.695 35.0 1.38 Comparative Example 20 Electron beam Continuous linear 0.060 0.040 0.020 0.695 35.0 1.25 Example 21 Electron beam Continuous linear 0.060 0.030 0.030 0.695 35.0 1.25 Example 22 Electron beam Continuous linear 0.060 0.020 0.040 0.695 32.0 1.23 Example 23 Electron beam Continuous linear 0.060 ?0.015 0.075 0.690 31.0 1.23 Example 24 Electron beam Continuous linear 0.120 0.020 0.100 0.690 31.0 1.23 Example 25 Electron beam Continuous linear 0.120 ?0.080 0.200 0.695 35.0 1.25 Example 26 Electron beam Continuous linear 0.160 ?0.090 0.250 0.695 35.0 1.25 Example 27 Electron beam Continuous linear 0.180 ?0.120 0.300 0.695 35.0 1.25 Example 28 Electron beam Dot 0.070 0.070 0.000 0.690 32.0 1.36 Comparative Example 29 Electron beam Dot 0.070 0.050 0.020 0.690 32.0 1.23 Example 30 Electron beam Dot 0.070 0.040 0.030 0.690 32.0 1.23 Example 31 Electron beam Dot 0.070 0.030 0.040 0.685 30.0 1.20 Example 32 Electron beam Dot 0.070 ?0.005 0.075 0.685 30.0 1.20 Example 33 Electron beam Dot 0.140 0.040 0.100 0.685 30.0 1.20 Example 34 Electron beam Dot 0.140 ?0.060 0.200 0.690 32.0 1.23 Example 35 Electron beam Dot 0.170 ?0.080 0.250 0.690 32.0 1.23 Example 36 Electron beam Dot 0.190 ?0.110 0.300 0.690 32.0 1.23 Example 37 Laser Continuous linear 0.010 0.030 ?0.020 0.705 50.0 1.45 Comparative Example 38 Laser Dot 0.020 0.070 ?0.050 0.700 45.0 1.40 Comparative Example 39 Electron beam Continuous linear 0.020 0.040 ?0.020 0.700 50.0 1.45 Comparative Example 40 Electron beam Dot 0.030 0.080 ?0.050 0.695 45.0 1.40 Comparative Example

[0122] According to Table 2, the effects of reducing noise and reducing building factor can be confirmed, regardless of the energy beam source and the irradiation form, under the conditions of Nos. 2 to 9, 11 to 18, 20 to 27, and 29 to 36 where the ?AB is positive (exceeding 0.000%), compared to Nos. 37 to 40 where the ?AB is negative. Especially, good effects can be confirmed under the condition where the ?AB is 0.040% or more and 0.200% or less. Better effects can be confirmed under the condition where the ?AB is 0.050% or more and 0.150% or less.